First of all, 赤い、白い、青い、黒い are all adjective. 「赤い車がある。」「私の髪は黒い。」 As the name of colors, which are nouns, we have 赤、白、青、黒。 赤、白、青、黒 are the only four colors that have adjectival form. These four colors were fundamental colors when Japanese color names were born. Plus, 黄色and 茶色 can made into adjective（黄色い、茶色い）only with 色。We don't say 黄いnor 茶い。 Other color names are all nouns; there is no adjectival form. So you need to use them as nouns grammatically.「私の車は緑です」「緑の車がほしいです。」 You can put 色 after all names of colors. So you can say 赤色、白色、青色、黒色、緑色 as well as 銀色、金色、茶色 and オレンジ色. For some common colors, we just drop 色. So it's ok to say 銀、金、茶、オレンジ. However, when the color name was made from actual material color (such as 銀色・金色--they are "the color of silver" and "the color of gold") it may not clear if you are talking about the material or the color. For example, if you say 「私の車は金です。」, we assume you mean its color because usually cars are not made of gold. If you say 「私の指輪は金です。」we would assume you mean the material, not the color. In this case, if you want to say the color is gold(but material may not be gold), you put 色 .「私の指輪は金色です。」 >By the way, オレンジ is English --- any reason not to just say 橙(色)? I think it's just because English words sound cooler than traditional Japanese names.